1. Field of the Invention
The present invention relates to an image sensing apparatus and imaging system.
2. Description of the Related Art
An image sensing apparatus includes a pixel array in which a plurality of pixels are arrayed in the row and column directions, and a readout unit which reads out a signal from the pixel array.
According to a technique disclosed in Japanese Patent Laid-Open No. 2005-217158, a column amplifier (operational amplifier) for amplifying a signal read out from a pixel on each column of the pixel array, and an accumulation capacitor for holding a signal amplified by the column amplifier are arranged in the readout unit for each column of the pixel array. This technique increases the output resistance of the column amplifier in an arrangement in which the column amplifier and accumulation capacitor are connected to a common ground supply line. According to Japanese Patent Laid-Open No. 2005-217158, the voltage of the ground supply line hardly drops even upon variations in the operating current of the column amplifier.
However, according to the technique disclosed in Japanese Patent Laid-Open No. 2005-217158, the column amplifier and accumulation capacitor are connected to a common ground supply line. When a bright object is captured, a pseudo signal may be mixed in a pixel output.
More specifically, the column amplifier is an operational amplifier formed from a plurality of MOS transistors. The column amplifier includes an input transistor having a gate electrode and a back-gate electrode which receives a signal. The accumulation capacitor includes the first electrode for holding a signal, and the second electrode which faces the first electrode. In the technique disclosed in Japanese Patent Laid-Open No. 2005-217158, the back-gate electrode of the input transistor and the second electrode of the accumulation capacitor are connected to a common ground supply line.
In this case, when a bright object is captured, a large signal is read out from a predetermined pixel in the pixel array of the image sensing apparatus, and held in the first electrode of the accumulation capacitor. As the potential of the first electrode fluctuates, the potential of the second electrode capacitively coupled to the first electrode also fluctuates, and the potential of the ground supply line fluctuates transiently. The transient response of the ground supply line influences the column amplifier, as shown in FIG. 8. Since the voltage of the back-gate electrode of the input MOS transistor of the column amplifier fluctuates, the column amplifier is likely to mix a pseudo signal in an input signal. For example, when the image of a bright part of an object which is dark in the entire frame is captured, a pseudo signal is superposed in a pixel output from a row corresponding to the bright part under the above-mentioned influence via the ground supply line.